Original article Optimisation of vacuum frying of gold kiwifruit slices: application of response surface methodology Lemuel M. Diamante,* Geoffrey P. Savage & Leo Vanhanen Department of Wine, Food and Molecular Biosciences, Lincoln University, Lincoln, Canterbury, New Zealand (Received 9 May 2011; Accepted in revised form 19 October 2011) Summary Response surface methodology was used to investigate the effects of the level of maltodextrin (MD), frying temperature and time on the moisture, colour and texture properties of the vacuum-fried gold kiwifruit slices and to determine the optimised conditions for vacuum frying. The moisture content of vacuum-fried gold kiwifruit slices decreased with increasing frying temperature and frying time. The colour change of the product increased with increasing frying temperature. The browning index of the product increased with increasing frying temperature and frying time. The breaking force of the product gave higher values when processed at middle range of frying temperature and MD level. When processing vacuum-fried gold kiwifruit slices, there was a need to use frying temperatures of 72.0–76.3 °C, frying times of 35.0–65.0 min and an MD level of 40% to achieve products with acceptable moisture, colour and texture properties. Keywords Colour, gold kiwifruit, moisture, response surface methodology, texture, vacuum frying. Introduction Vacuum frying is the technique of deep-fat drying foods under pressures well below atmospheric levels, preferably below 6.65 kPa, which serves to reduce oil content, discolouration and losses of vitamins and other compounds normally associated with oxidation and high temperature processing (Garayo & Moreira, 2002). It has been used for different foods, including pineapple, apples, carrots, blue potato, sweet potato, beans, mangoes, kiwifruit and jackfruit (Fan et al., 2005a,b; Bouchon & Mariscal, 2008; Da Silva & Moreira, 2008; Perez-Tinoco et al., 2008; Diamante, 2009; Diamante et al., 2011). The main factors influ- encing the fried products are the time and temperature combinations of the cooking process; the correct combination is essential in producing a food product with acceptable physical attributes (such as colour, appearance, texture and flavour) as well as preserving nutritional, but unstable, compounds such as vitamin C and carotenoids. Kiwifruit are well known to contain high levels of fibre, minerals, vitamins, antioxidants, phenolic com- pounds and other bioactive substances that have the effects of improving digestive health, increasing and maintaining immunity, protecting the body against oxidative stress that could lead to health problems and disease and contributing to overall well-being (Farr et al., 2008; Shu Molan et al., 2008; Skinner et al., 2008). Their abundance in New Zealand and their economic importance leads to kiwifruit products being both cost-effective and high-earning, making them a good candidate for the development of new health food products, including functional foods (Zespri Group Limited, 2009). There are two well-known cultivars, the green kiwifruit (Hayward) and the gold kiwifruit (Hort16A). Green kiwifruit has a vibrant green fleshed fruit, with a small white core and black seeds. Gold kiwifruit has a smooth hairless skin that is bronze in colour, a golden flesh interior, a white core and black seeds (Martin, 2003). Preliminary vacuum frying studies on the two culti- vars have shown that the green kiwifruit did not retain its green colour while the gold kiwifruit gave attractive gold kiwifruit product. Hence, it was decided to concentrate on the gold kiwifruit for further vacuum frying studies. Maltodextrin (MD) is a common food additive made by the hydrolysis of starch and comes in the form of a white powder and has a sweet taste. It is commonly used as a bulking agent for artificial sweeteners and as a drying aid (Wang & Wang, 2000). Osmotic dehydra- tion through the use of salt, sugar and starch solutions is used to concentrate solids in foods. The driving force for water removal is the concentration gradient between the osmotic solution and the intercellular *Correspondent: E-mail: Lemuel.Diamante@lincoln.ac.nz International Journal of Food Science and Technology 2012, 47, 518–524 518 doi:10.1111/j.1365-2621.2011.02872.x Ó 2011 The Authors. International Journal of Food Science and Technology Ó 2011 Institute of Food Science and Technology